Ken, it seems that the only way it will come out without damaging your gun is to take it to a competent gunsmith. It should be a simple exercise, with a precision drill press, or milling machine to drill the screw out and to re-tap the hole. Of course, you would have to use an oversize screw--but it would do the job.
Another thing to try is to soak the part in Kroil. Clamp the gun with the screw up in a padded vise. Apply Kroil, and tap the sides of the frame with a plastic faced mallet a few times. Then, let it sit. Re-visit the frame every 12 or so hours; apply more Kroil and tap.
After about three or four days of this, get a close fitting screwdriver, seat it and tap the screwdriver a few times--fairly hard--with a ball pein hammer. (Like you're driving a stake). This should hopefully break the bond. Try to unscrew while applying force straight down on the screwdriver.
Good luck, and I hope you can work it out.